Self-biting presents a serious problem in male nonhuman primates that cannot be group-housed. Limited success has been experienced in reducing self-biting in singly-housed male rhesus macaques by enhancing their environment with enrichment devices. Because sexual and aggressive behavioral problems in men have been treated with the antiandrogen, cyproterone acetate (CA), the efficacy of CA was tested on reducing self-aggression in eight singly-housed adult male rhesus macaques. Baseline behavior was recorded for 14 weeks, followed by treatment with 50 mg CA/wk for 8 weeks, then 100 mg CA/wk for 4 weeks, and an additional 4 weeks of observation after treatment cessation. A repeated measure of variance analysis (P = 0.5) was used to analyze results. The main findings during CA treatment were (1) the occurrence frequency of self-biting and some other abnormal behaviors was significantly reduced, but not eliminated; (2) blood levels of testosterone were significantly reduced; (3) but the frequency of masturbation and sperm counts did not drop. It was concluded that CA is a partially effective long-term treatment for adult male rhesus macaques with behavioral problems that include self-destructive actions. The findings suggest that self-directed aggressive behavior is modulated by testosterone and/or other factors.